Water heater control system with thermal retarder



May 21; 1946. c. M. OSTERHELD WATER HEATER CONTROL SYSTEM WITH THERMAL RETARDER.

Filed'Fb. 21, 1944 2 Sheets-Sheet l I I 1 l I I I I l/ I/ I ll l I I I I II l 1 ll l I I I I I ATTORNEY Patented May 21, 1940 WATER HEATER CONTROL SYSTEM WITH THERMAL RETARDED Clark M. Osterheld, Stougliton, Wis asalgnor to .McGraw Electric Company, Elgin, Ill., a corporation of Delaware Application February 21, 19, Serial No. 523,323

1'7 Claims.

My invention relates to electric heating and particularly to water heater control systems including a thermal retarder.

An object of my invention is to provide a water heater control system that shall cause energize.- tion of an electric heater for a hot water tank immediately in case of use of a relatively large quantity of hot water and that shall cause reener gization of a heater with a preset period of time delay in case of use of a relatively small quantity only of hot water from the tank.

Another object of my invention is to provide a control system for a tankheater that shall include a thermal retarder heater control switch unit of specificdesign and construction .to effect immediate energization of a tank heater upon start of an oil-peak period in case the tank contains a relatively large amount of cold water and to eifect energization of the heater with a preset time period of delay in case only a relatively small amount of cold water is in the tank at the start of an off-peak period.

Other objects of my invention will either be apparent from a description of a device and system embodying my invention or will be pointed out hereinafter and set forth particularly in the appended claims.

In the drawings.

i ure 1 is a vertical sectional view through a 7 hot water tank showing schematically the positions of a thermal retarder switch unit, a thermal heater control switch, and a water heating unit.

Fig. 2 is a diagram of the electric circuit showing the connections used in my improved control system,

Fig. 3 is a plan top view of the device shown in Fig. 4,

Fig. 4 is a vertical sectional view taken on the line's-l of Fig. 3, the position of the contacts being that shown in Fig. 2,

Fig. 5 is an end view of the parts shown in Fig. 4, and, I

Fig. 6 is a sectional view taken on the line 8-4 of Fig. 4. v

Referring first of all to Fig. l of the drawings I have there shown a usual domestic hot water tank ll provided with a lower cold water inlet p p I! and an upper hot water outlet pipe I! as well as with a heat-insulating covering I! therearound, which covering is held in proper operative position by an outer casing II. I provide also supports 2 I, 'all in a manner well known in the art. Iprovide preferably, but not necessarily, 'a single electric heater 2! which is preferably placed near the lower end of the tank [I and II which may be positioned in a tunnel 2!, all in a manner well known in the art.

Referring now to Fig. 2 of the drawings I have there shown a diagram of connections of the electric circuit used in my improved control system and in the th rmal retarder heater control switch unit comprising a part thereof. I provide a continuously' operative timer 21 which includes an electric motor 28 having a worm St on the.

end of the shaft, which worm is adapted to engage with a worm gear 33. This worm gear is mounted on one end of a shaft 35 and a cam disc 31 is mounted on the other end of said shaft.

While no hearings or other I fixed supporting. means are shown I desire it to be understood that.

any suitable or desired hearing system may be provided. It is to be understood that the design, construction, and adjustment of the timer is such that the cam disc 3i will be caused to rotate through one complete turn. for each twenty-four hour day.

The cam disc 8? has a uniform radius over the on-peak portion 38 thereof; but theoff-peek portion M has a slightly larger radius. It is to be understood that the peripheral extent of the portion M is substantially equal to that of the off-peak period and that the position of the cam disc 31 :on the shaft "is such that the portion M will engage a circuit control switch during the oil-peak period, which may be considered, for

illustrative purposes only. as extending from twelve midnight to six 9.. m. or generally, during late night hours.

The circuit control switch includes a substantially resilient contact arm ts having 9. lug 4B of electric-insulating material at its outer end, the arm 43 being adapted to be moved into enga ment with a substantially rigid contact arm 4! by cam portion H. The contact arm 53 is normally biased out of engagement with contact arm 41. The two contact arms are shown as bein suitably held in proper operative positions relative to each other and to the cam disc by blocks 49 of electric-insulating material. Reference to Fig. 2 will indicate that lug l! is just about to engage the portion ll of greater radius, it being understood that the cam disc 31 is caused to rotatein a clockwise direction; hence the time is 55 with one contact terminal 51 of a thermal retarder switch unit designated by numeral 59. A second contact terminal BI 01' the thermal retarder 50 i connected by aconductor 63 with one terminal of electric heater 23, the other terminal An expansion rod 1I extends into the tube 69 and is adapted to engage with a lug 13 of electricinsulating material which is mounted on the free or movable end of a substantially resilient contact arm 15, whereby the latter may be moved into and out of engagement with the substantially rigid contact arm 65 suitably mounted on the outside surface of the tank II. Block 11 o! electric-insulating material may be used to support the two contact arms 65 and 15. Contact arm 15 is electrically connected by a conductor 19 with supply circuit conductor 53.

Referring now to Figs. 3 to 6 inclusive I have there shown a form of thermal retarder heater control switch unit which constitute one of the essential elements of my improved heater control system. A block 8| of good heat-conducting qualities, having an inner surfaceof such shape as to engage in good heat-conducting relation with the outside arcuate surface of the tank, may be suitably secured thereagainst in any desired manner, as by welding eams 03. The outer flat surface of member 8| has secured thereagainst the flat inner surface of a heat-conducting member 05 which is provided with lateral ears or lugs 01 through which machine screw 09 mayv extend into threaded relation with member 0|.

The front surface of member 05 is provided with a recess 90 of substantially L-shape and is further provided with a removable cover member 9| of heat-conducting material, which i also of substantially L-shape, having a forwardly extending portion 93 at its lower end. Any suitable or desired means may-be provided for securing the cover member 0| against member 85.

I provide a first creep-type bimetal bar 95, the lower end of which is fixedly secured in a lower part of the cover member III, the bimetal bar 95 being normally substantially straight and extending substantially parallel to the inner in cllned surface of cover member 9|, as shown by the broken lines in Fig. 4.

I provide a support 01 of heat-insulating material, which support is 01' substantially inverted T-shape and is secured to portion 93 of cover member 0| by a plurality of machine screws 99. I provide further a cover member IOI for the front end portion of member 01 to protect a second creep-type bimetal bar I 03, the lower end of which may be fixedly held in a slot in one side portion or member 91 in substantially the position shown by the full lines in Fig. 4. Cover member IllI may be held in proper operative position by a machine screw I05. The free end of the second bimetal bar I03 is adapted to engage a proiection on the front surface of member 91 when cold and will flex into the positionshown by the broken lines when heated.

I provide a snap acting switch member adapted to be moved from one to the other of its two operative positions by the bimetal bars 95 and I03 respectively. This switch includes a lower toggle arm I01 pivotally supported by a pivot pin I09 in a pair oi upstanding lugs III constituting a part of the left hand arm of member 91.

I provide a second toggle arm H3 which is pivotally connected with the upper end of toggle arm I01 as by Divot pins II5. I provide a contact bridging member II1 insulated y mounted against the left hand surface of arm II3, which contact bridging member i adapted to engage with and be disengaged from contact terminals 51 and BI suitably secured against the upper end portion of the stem of the member 91 of T-shape as by machine bolts HI. I provide also a stop member I23 which may be adjusted in the upper end portion of the stem of member 91. The two toggle arms are connected by an over center spring I25 connected with the toggle arm-5 intermediate their respective ends.

I provide a substantially rectangular frame I21 which is also engaged by pivot pins H5. The first bimetal bar 95 has an opening therethrough adjacent it upper free end through which there extends a rod I29 connected to fram I21 by means of which flexing movement of bimetal bar 95, upon change of temperature, is communieated to frame I21 and thereby to the snap acting switch. I provide a pair of nuts I3I and I33 on each side of bimetal bar 95 which may be adjusted on the screw threaded end portion of rod I29 to determine the temperatures at which the switch will operate.

Bimetai bar I03 is also provided at its upper end with an opening,.through which extends a connecting rod I35, having its inner end connected with frame I21. A double nut I31 is provided on the outer screw threaded end of c0nnecting rod I35 and is located on the outside of bimetal bar I03.

The thermal retarder switch unit, shown more particularly in Figs. 3 to 6 inclusive, is preferably mounted against the outside of the tank at about the half height of the tank, substantially as shown in Fig. 1 of the drawings, in an opening provided therefor in the heat-insulation, a cover member I30 being provided to suitably cover the opening. The Iver thermally-actuable heater control switch 81 may be located in an opening in the heat-insulation, which may be covered by a cover member I4 I.

The first bimetal bar 95 is adapted to be subjected to the temperature of the water in the tank through the member 8|, casing 85, and cover 9|. The position of bimetal bar 95 when cold is that shown by the broken line in Fig. 4, while its position when at the maximum operating temperature, on the order of F., is that shown by the full lines. When the temperature of the bimetal bar 95 is on the order of 150 F., the switch will be in the open position as shown in Fig. 4 of the drawings.

The second bimetal bar I03 may be caused to flex in a clockwise direction from the cold position, shown by the full; lines in Fig. 4, by a small heater coil I43, the terminal portions of which may be suitably connected to contact terminals I45 secured against the lower arm of member 31 and is made stronger than bar 95.

The heater I43, which is insulatedly mounted on bimetal bar I03, has one terminal thereof connected to rigid contact arm 05 by a conductor I41, while the other terminal thereof is connected by a conductor I49 with one terminal of an ad- Justable resistance iii, the other terminal of which is connected to conductor 55.

When the temperatures of the two bimetal bars are at ordinary room temperature value, on the order of 70 F., the first bimetal bar will be in the position shown in broken lines in Fig. 4, and the second bimetal bar will be in the position shown in full lines in Fig. 4, so that the switch will be in closed position. If now the first bimetal bar is subject to hot water in the tank. it will be in the position shown in full lines in Fig. 4, and if the heating coil I43 has been energized appreciably less than a predetermined length of time, the switch will be in open position, as shown in Fig. 4. If now, with the first bimetal bar still subject to hot water having a temperature on the order of 150 F., the second bimetal bar has been heated to a temperature on the order of 300 F.,

the second bimetal bar will be in the position shown by the broken lines in Fig. 4, so that the switch will again be in closed position. If the heater 23 has been deenergized for from five to ten hours and the tank is still full of hot water, the temperature of both bimetal bars will be on the order or 150 F., or slightly lower, and the adjustment of the thermal retarder unit is such that the switch is still in open position.

While I prefer to make the watt input of the coil I43 sufilcient to cause flexing thereof to a sufficient degree to cause snap-over action of the two toggle arms and of the spring I!!! from the position shown in Fig. 4 of the drawings to the position, particularly of the upper end portion of toggle arm II3, shown by the broken lines, in five hours, the use of an adjustable rheostat IBI may either lengthen or shorten said operating time, u

I wish to point out here that the two bimetal bars 95 and I03 are so selected that they are dissimilarly thermally responsive to predetermined change of temperature. Thus the first bar 98 may be thinner and narrower than the second bar I03, or it may be of material having greater deflection characteristics than the second bar I03, so that the second bar must be heated to a higher temperature than the first bar to cause it to deflect through the same distance, which higher temperature may be on the order of 300 F.

Let it now be assumed that the tank I I has just been filled with cold water in order to start the system, and that the timer ha been so adjusted or manipulated that contact arms 43 and 41 are in engagement with each other, Under these conditions the temperature of bimetal bar 05 will be substantially that of the cold water in the tank II, which, for illustrative purposes, maybe considered to be 70 F. The two contact arms 33 and 15 of the thermally-actuable heater control switch 81 will also be. in engagement since the length of expansion rod ll will be a minimum. Under these conditions a small current will fiow through coll I43, but bimetal bar 05 will be in the position shown by the broken lines of Fig. 4 of the drawings, with the result that rods I29 and I3! and the other parts secured thereto wlllbe moved to the. right so that contact bridging member I" will be in engagement with contact terminals 51 and 6| and heater 23 will be energized because of the closure of the thermally-actuable heater control switch 31, of the electric circuit switch actuated by the continuously operative timer as well as the engagement of contact bridging member Ill with terminals 31 and SI of the thermal retarder heater. control switch. This will start heating of the water in the tank, which will continue without interruption even after bimetal bar I03 has been heated at the end of a certain preset time and moved to the position shown in broken lines in Fig. 4, this flexing having no eifect upon the switch since the initially cold bimetal bar 33 has moved the switch to its closed position. When heating a tank full of cold water by a single electric heater located near the lower end of the tank, the heated water is distributed substantially uniformly throu hout the length of the tank, that is the temperature of the water in the tank is substantially the same from top to bottom and increases as the water is heated. Even though bimetal bar 35 tends to fiex in a counterclockwise position when heated, it and the toggle-arms and rods I29 and I35 will be held in positions that will keep the switch closed by the stronger bimetal bar I03 as it is heated by coil I43.

Energization of the heater 23 will therefore continue until substantially all of the water in the tank has been heated to say 150 F., when rod 11 willhave expanded enough to cause opening of the lower thermal switch 31 by reason of movement of outer arm 15 away from the inner arm 05. When this switch opens, which it does when substantially all of the water in the tank is hot, energization of the heater 23 will be terminated. Bimetal bar I03 will then begin to flex in a counterclockwise direction, permitting movement of rods I35, I29, and the parts connected thereto in a left hand direction because of the tendency of the first bimetal bar '83 to return to its hot position, shown by the full lines of Fig. 4. Contact bridging member will be moved out of engagement with contacts 51 and 8| and into the osition shown in full lines in Fig. 4.

Let it now be assumed that a relatively small quantity of hot water is withdrawn from the tank, this amount being such that tube 69 will be subjected to cold water. This causes engagement of contact arms 65 and II, but this will have no effect as to immediate energization of heater 23.

Let it be assumed that the continuously operative timer 2! has moved the cam disc 3'Ito the position shown in Fig- 2 of the drawings, so that at the start of an off-peak period, contact arm 43 will, be moved into engagement with contact arm 41 to cause closing of the circuit switch and resultant energization of heater I43 on bimetal bar I03. After the passage of the predetermined time delay period or say five hours, bimetal bar I08 will have been heated enough to flex sufficiently to cause closing movement of the snap-acting thermal retarder switch so that then energization. of

' the heater 23 will occur with heating up of the cold water then in the tank.

Let it be supposed, however, that a relatively large quantity of water was withdrawn from the tank during the on-peak period so that the switch in the thermal retarder was in its closed circuit position because of the fact that bimetal bar was in its cold position. In this case energization of heater'23 would start immediately up n engagement of contact arms 43 and 41 caused by the timer at the start of an oil-peak period.

It is evident that the adlustably preset time of heating up of the bimetal bar I03 should be such that substantially all of the cold water in the tank will be heated to the desired maximum temperature before the end oi! the off-peak period, and where I have mentioned a delay period of time on the order of five hours, this may be shortened as may be found necessary. It substantially all of the water in the tank is heated before the end of the oil-peak period, the opening of the lower thermally-actuableswitch 01 will cause deenergization of the heater 23, but if less than substantially all or the water in the tank has been heated to a predetermined maximum temperature before the expiration of the cit-peak period, the opening of the circuit switch by movement of resilient contact arm 43 out of engagement with contact arm 41 will cause deenergization not only of the tank heater 23, but also or the bimetal bar heater It is therefore obvious that my invention pro vides a water heater control system effective to permit immediate energization of the tank water heater at the start of an oil-peak period in case a relatively large quantity of cold water is in the tank or to delay energization of the tank heater for an adjustably predetermined time delay period in case only a relatively small quantity of cold water is in the tank at the start ofan offpeak period.

Various modifications may be made in the de vice and system embodying my invention without departing from the spirit and scope thereof, and I desire that all modifications coming within the appended claims shall be considered a part of my invention.

I claim as my invention:

1. A water heater control system for a. hot water tank having an electric heater, comprisin a thermal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature of the water in the tank, a second bimetal bar, a heat-insulating support for said second bar supported by said heat-conducting support, an electric heating coil for said second bimetal bar and a control switch for said electric heater supported by said heat-insulating support, mechanical connections between said switch and both said bimetal bars, said switch being actuable mechanically by said first bimetal bar into heater-energizing position when said two bimetal bars are at substantially ordinary room temperature.

2. A water heater control system for a hot water tank having an electric heater, comprising a thermal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature or the water in the tank, a second bimetal bar, a heat-insulating support for said second bar supported by said heat-conducting support, an electric heating coil for said second bimetal bar and a control switch for said electric heater supported by said heat-insulating support, mechanical connection between said switch and both said bimetal bars, said switch being actuable mechanically by said first bimetal bar into heater-energizing position when said two bimetal bars are at the same relatively low tern perature.

3. A water heater control system for a hot water tank having an electric heater comprising a thermal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature of the Water in the tank, a second bimetal bar, a heat-insulating support for said second bar supported by said heat-conduct ing support, an electric heating coil for said second bimetal bar, and a control switch for said electric heater supported by said heat-insulating support actuable by said second bimetal bar into heater-energizing position when said two bimetal bars are at predetermined relatively high temperatures.

4. A water heater control system tor a hot we ter tank having an electric heater, including a termal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature oi! the water in the tank, a second bimetal bar, a heat-insulating support for said second bar supported by said heat-conducting support, an electric heating coil for said second bimetal .bar, and a control switch for said electric heater supported by said heat-insulating support, mechanical connections between said switch and both said bimetal bars, said switch being actuable by said first bimetal bar mechanically into heater-energizing position when said first bimetal bar is subject to the temperature of cold water in the tank and said second bar is at substantially the same temperature.

5. A water heater control system .for a hot water tank having an electric heater, including a thermal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted. to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature of the water in the tank, a second bimetal bar, a heat-insulating support for said second bar supported by said heat-conducting support, an electric heating coil for said second bimetal bar, and a control switch for said electric heater supported by said heat-insulating support, mechanical connections between said switch and both said bimetal bars, said switch being actuable by said first bimetal bar mechanically into heater-energizing position when said first bimetal bar is subject to the temperature of cold water in the tank and said second bar is at substantially ordinary room temperature and being actuable by said first bimetal bar into heater-deenergizing position when said first bar is subject to the temperature of hot water in the tank and said second bar is at substantially ordinary room temperature.

6. A water heater control system for a hot water tank having an electric heater, including a thermal retarder switch unit comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be mounted on a hot water tank intermediate the ends thereof to be subject to the temperature of the water in the tank, a second bimetal bar, a heat-insulating supp t for said second bar supported by said heat-conducting support, an electrical heating coil for said second bimetal bar, and a control switch for said electric heater supported by said heat-insulating support actuable by said first bimetal bar into heater-energizing position when said first bimetal bar is subject to the temperature of cold water in the tank and said second bar is at substantially ordinary room temperature, actuable by said first bimetal .bar into heater-deenergizing position when said first bar is subject to the temperature of hot water in the tank and said second bar is at substantially ordinary room temperature and actuable by said second bar into heater-energizing position with a predetermined timedelay period 'by said second bimetal bar when said heating coil has been energized for a predetermined length of time and said first bimetal bar is subject to the temperature of hot water in the tank.

7. A thermal retarder switch unit for controlling the energization 01 an electric heater for a hot water tank, comprising a first bimetal bar, a heat-conducting support for said first bar adapted to be secured to a hot water tank in good heat-conducting relation therewith intermediate the ends thereof, a second bimetal bar, a heat-insulating support for said second bar to hold it spaced irom said first bar, an electric heating coil on said second bar and a heatercontrol switch supported by said heat-insulating heating coil thereon has been energized for a predetermined length of time.

8. A water heater control system for a hot water tank having an electric heater at its lower end, a thermal heater control switch subject to tank water temperature at the lower end of the tank and a thermal retarder switch unit adapted to be mounted in heat-receiving relation with the water in the tank and comprising a first bimetal bar, a heat-conducting support therefor adapted to be secured on a tank intermediate the ends thereof, a second bimetal bar, a heat-insulating support therefor adapted to be secured to and supported by said heat-conducting support, an

electric heating coil for said second bar and a.

snap-acting heater-control switch supported by said heat-insulating support electrically connected in series circuit with said lower thermal switch and adapted to be moved into heater-energizing position by said first bimetal bar when it is subject to the temperature of cold water in v the tank and saidsecond bar is at substantially ordinary room temperature.

9. A water heater control system for a hot water tank having an electric heater at its lower end, a thermal heater control switch subject to tank water temperature at the lower end of the tank and a thermal retarder switch unltadapted to be mounted in heat-receiving relation with the water in the tank and comprising a first bimetal bar, a heat-conducting support therefor adapted to be secured on a tank intermediate the ends thereof, a second bimetal bar, a heatinsulating support therefor adapted to be secured to and supported by said heat-conducting support, an electric heating coil for said second bar and a snap-acting heater-control switch supported by said heat-insulating support electrically connected in series circuit with said lower thermal switch, mechanical connections between said snap-acting switch and both said bimetal bars, said switch being adapted to be moved into heater-energizing position mechanically by said first bimetal bar when it is subject to the temperature of cold water in the tank and said second bar is at substantiallyv ordinary room temperature and being movable into heater-deenergizing position by said first bar when-said first bar is subject to the temperature of hot water in the tank and said second bar is at substantially ordinary room temperature.

10. A water heater control system for a hot water tank having an electric heater at its lower end, including a thermal heater control switch subject to tank water temperature at the lower end of the tank and a thermal retarder switch unit adapted to be mounted in heat-receiving relation with the water in the tank and comprising a first bimetal bar, a heat-conducting upport therefor adapted to be secured on a tank intermediate theends thereof, a second bimetal bar, a heat-insulating support therefor adapted to be secured. to and supported by said heat-conducting support, an electric heating coil for said second bar and a snap-acting heater-control switch supported by said heat-insulating support electrically connected in series circuit with said lower thermal switch, mechanical connections between said snap-acting switch and both said bimetal bars, said switch being adapted to be moved ond bar is at substantially the same temperature,

to be moved into heater-deenergizing position by said first bar when said first bar is subject to the temperature or hot water in the tank and 'said second bar is at substantially ordinary room temperature and is movable into heater-energizing position by said second bar when said first bar is subject to the temperature of hot water in the tank and the heating coil on said second bar has been energized for a predetermined length of time.

11. An off-peak water heating system for a hot water tank having an electric heater, a lower cold water inlet and an upper hot water outlet, said system comprising a continuously operative timer, a first heater control switch adapted to be moved into and held in closed position by said timer during an off-peak period, a second heater control switch in series circuit with said timer controlled switch subject to tank water temperature at the lower portion of the tank and adapted to be moved into closed position when subject to cold water, a third heater control switch in series electric circuit with said first and second switches and means to actuate said third switch comprising a first bimetal bar adapted to be mounted on a tank intermediate the ends thereof in heat-receiving relation to the water in the tank, a second bimetal .bar heat-insulated from the tank and supported thereby, an electric heating coil for said second bar controlled jointly by said second switch and by said timer-controlled switch to be energized at the start of an off-peak period, said three switches being jointly effective to cause energization of the tank heater'with a predetermined time period of delay after closing of the timer-controlled switch in case said second heater control switch only is subject to cold Water in the tank.

12. A control system as set forth in claim 10 and including means for varying said predetermined length of time.

13'.-A control system as set forth in claim 10 and including manually actuable means for varying said predetermined length of time.

14. A control system as set forth in claim 11 I and including means for varying said predetermined time period of delay.

15. A control system as set forth in claim 11 and including manually actuable means for varying said predetermined time period of delay.

15 An off-peak water heatingsystem for a hdt water tank having an electric heater, a lower cold water inlet and an upper hot water outlet, said system comprising a continuously operative timer, a first heater control switch adapted to be moved into and held in closed position by said timer during an off-peak period, a second heater control switch in series circuit with said timercontrolled switch subject to tank water temperature at the lower portion of the tank and adapted to be moved into closed position when subject to cold water, a third heater control switch in series electric circuit with said first and second switches and means to actuate said third switch comprising a first bimetal bar adapted to be mounted on a tank intermediate the ends thereof in heat-receiving relation to the water in the tank, a second bimetal bar heat-insulated from the tank and supported thereby, anelectrlc heating coil for said second bar controlled jointly by said second switch and by said timercontrolled switch to be energized at the start of an oil-peak period, said three switches being jointly effective to cause energization oi the tank heater with a predetermined time period of delay after closing of the timer-controlled switchv in case said second heater control switch only is subject to cold water in the tank, and to cause energization of the tank heater immediately upon closing of the timer-controlled switch in case the second and third heater control switches are both subject to cold water in the tank.

17. An off-peak water heating system for a hot water tank having an electric heater, a lower cold water inlet and an upper hot water outlet, said system comprising a continuously operative timer, a first heater control switch adapted to be moved into and held in closed position by said timer during an ofl-peak period, a second heater contro1 switch in series circuit with said timercontrolled switch subject to tank water temperature at the lower portion 01' the tank and adapted to be moved into closed position when subject to cold water, a third heater control switch in series electric circuit with said first and second switches and means to actuate said third switch comprising a first :birnetal bar adapted to be mounted on a tank intermediate the ends thereof in heat-receiving relation to the water in the tank, a second bimetal bar heat-insulated from the tank and supported thereby, an electric heating coil for said second bar controlled Jointly by said second switch and by said timer-controlled switch to be energized at the start of an off-peak period, said three switches being jointly eflective to cause energization of the tank heater with a predetermined time period of delay after closin! of the timer-controlled switch in case said second heater control switch only is subject to cold water in the tank, and to cause energization of the tank heater immediately upon closing of the timer-controlled switch in case the second and third heater control switches are both subject to cold water in the tank, and said second switch being effective to cause deenergization of said tank heater it substantially all of the water in the tank has been heated to a predetermined high temperature before the end of an oil-peak 30 period.

CLARK M. OSTERHEID. 

